Entner-ALUMINIUM-2007

genisim

A L U M I N I U M S M E L T I N G I N D U S T R Y

AlPrg: a software tool for aluminium smelting

P. M. Entner, Sierre

AlPrg is a collection of computer

programmes (modules) for PCs

using interactive graphics to display

the results of calculations

concerning aluminium production

by electrolysis. The user applies

these modules among other

things to visualise how varying

the parameters will affect pot

operation. He can then choose

the best values to operate the

electrolytic cells of a potroom so

as to optimise the most important

technical and economic results.

This paper reports on the use of

one of these modules, namely the

Electrolyte Properties Module.

This module calculates the physical

and chemical properties of the

cryolite electrolyte as a function

of its composition and temperature.

The Electrolyte Properties

Diagram Pane shows plots of these

bath properties in function of the

electrolyte parameters. This plot

predicts the behaviour of the bath

after a parameter change, and so

identifies the optimal bath composition

and bath temperature to

operate the electrolytic cells.

AlPrg was developed with the aim of

helping people occupied with aluminium

production by electrolysis. AlPrg

contains several modules that the

user selects depending on the tasks

he wants to study. For instance, if he

wants to know how much aluminium

a potroom is producing in one month,

then he uses the aluminium production

module. If he needs to find the set

value of the pot voltage, the alumina

feeder setting or how much aluminium

fluoride to add, he will apply the

corresponding, obviously more complex,

modules of AlPrg. Finally if the

user wants to improve the economic

performance of the aluminium smelting

plant, the profitability analysis

modules will help him to determine

those operational parameters to run

the plant in a better way.

This paper deals with the electrolyte

properties module of AlPrg.

All illustrations: Entner

Fig. 1: Layout of the User Interface of AlPrg. The figure shows 3 tab groups with the calculation

pages Technical Profitability Analysis, Aluminium Production and Pot Parameters

being selected. Two diagram panes namely the Pot Voltage Diagram Pane and the Electrolyte

Composition/Properties Diagrams Pane are docked on the programme user interface.

This module shows the physical and

chemical properties of the cryolite

electrolyte, like the liquidus temperature

or electrical conductivity. AlPrg

determines these values in relation

to the chemical composition and the

temperature of the electrolytic bath.

The electrolyte properties diagram

pane shows diagrams of these bath

properties in function of various plots.

These plots predict the behaviour of

the bath after a parameter change and

so help to determine the optimal bath

composition and bath temperature.

Programme layout

When the author of this paper worked

with Alusuisse and later with Alcan he

developed ElysePrg [1] a precursor of

AlPrg. AlPrg is, however, a completely

new concept for this application, especially

regarding the program features

and program layout. AlPrg consists

of pages and diagram panes. In the

windows the user changes numerical

values in input fields, and then AlPrg

calculates the corresponding results.

The diagrams (plots) show graphical

representation of properties depending

on various parameters. Formed as

graphical user interfaces (GUIs), the

plots allow the user to change values

on the plot by dragging their size representation

with the mouse pointer.

AlPrg calculates the corresponding

new resulting values and updates the

relevant pages and plot panes.

The user selects pages or plot

panes by clicking on the corresponding

menu or tab items. He can arrange

several pages on the programme user

interface and see them simultaneously

so as to study the calculation

results. The diagram panes can float

on the computer screen or they can

be docked, i.e. added on the AlPrg

user interface. The next figure (Fig. 1)

depicts an example of the AlPrg user

interface containing three tab groups

and two diagram panes. From the tab

groups the user has selected the technical

profitability analysis page, the

aluminium production page and the

pot parameter page. In addition he has

docked the pot voltage diagram pane

and the electrolyte composition/properties

pane on the user interface.

The user finds more information

about AlPrg in the conventional ac-

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user has clicked on the aluminium

fluoride concentration icon-plot, and

consequently the property plot shows

the liquidus temperature in dependence

of the aluminium fluoride concompanying

help system. He may also

consult the corresponding website [2]

that contains practically the same user’s

guide and theoretical background

information.

The Electrolyte Composition/

Properties Page

Fig. 2: Electrolyte Composition/Properties Page. In the upper part

of the page the user may change the concentration values in the

input fields. The lower part contains the property values. The

user may select the authors of the corresponding equations that

AlPrg uses to calculate the properties.

Fig. 2 shows the electrolyte composition/properties

page. In the upper

part of the page the user changes the

concentration values of the electrolyte

components of aluminium fluoride

(AlF 3

), calcium fluoride (CaF 2

), aluminium

oxide (alumina Al 2

O 3

), lithium

fluoride (LiF), magnesium fluoride

(MgF 2

) and potassium fluoride (KF).

The bath ratio is the weight ratio of

sodium fluoride over aluminium fluoride.

Aluminium oxide at anode effect

is the alumina concentration when

the anode effect occurs. The user can

suppress this parameter by inactivating

the check box.

The lower part of the page contains

the values of the electrolyte properties,

namely the liquidus temperature,

cosities of the electrolytic bath and of

the aluminium metal.

The bath temperature can be expressed

as the liquidus temperature

plus the superheat. The user selects

as input value either the superheat

(as shown in Fig. 2) or the electrolyte

temperature. Selecting a value as input

means that it is constant for the

following calculations (the superheat

in Fig. 2) and the other depending

value (the bath temperature in Fig. 2)

is calculated correspondingly.

To determine the liquidus temperature,

the electrical conductivity or the

densities the user can choose between

several relationships

published by

different authors.

Fig. 2 shows a Drop

Down Combo Box

where the user has

selected the equations

of Solheim [3].

He could alternatively

also click on

the names of Røstum

[4], Peterson

[5], Bullard [6], Lee

[7] or Dewing [8] to

choose the corresponding

relations.

The theoretical part of the AlPrg

website [9] contains all the relations

and equations that AlPrg uses for its

calculations.

The Electrolyte Composition/

Properties Plot

Fig. 4 shows the electrolyte composition/properties

plot pane together

with the electrolyte composition/

properties page.

Fig. 4: The Electrolyte Composition/Properties Diagram Pane

docked on the AlPrg user interface. Small icon-like plots

represent the selected electrolyte property (the liquidus

temperature in this figure) in function of the electrolyte

components. The user has selected the liquidus temperature

vs. aluminium fluoride content to be shown in the larger

property plot. The circles represent the current values of

the electrolyte composition.

the liquidus temperature depends on

these variables, the other parameters

staying constant. The small circles

represent these constant concentration

values shown on the electrolyte

composition/properties page.

Property plot

If the user wants to study an iconplot

more profoundly he clicks on

the icon-plot with the mouse pointer

and the icon-plot expands to replace

the larger property plot. In Fig. 4 the

Icon plots

Fig. 3: Drop Down Combo Box to select the authors of

the relations that AlPrg uses to calculate the liquidus temperature.

The user has chosen the equation of Solheim

[3] by clicking on the name with the mouse pointer.

the electrical conductivity, the maximum

alumina solubility, the total vapour

pressure, the densities and vis-

The idea of this plot pane

is to show a property of the

electrolytic bath in dependence

of the bath composition

and the bath temperature.

The plot pane of Fig.

4 consists of small icon-like

plots of the liquidus temperature

against the concentrations

of aluminium fluoride, calcium

fluoride, aluminium oxide etc. From

these plots the user can visualise how

Fig. 5: Liquidus temperature vs. Alumina Concentration

Property Plot. The user has clicked on the Al 2

O 3

icon-plot

and the property plot shows the liquidus temperature vs.

the alumina concentration. The gray areas are the regions

where the alumina concentration is lower than anode effect

concentration (on the left side) or higher than the maximum

alumina solubility (right side).

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centration between 0 and 30 weight

%. The circle represents the liquidus

temperature of 953.7 °C at the aluminium

fluoride concentration of 12

weight %.

Gray areas in the icon or property

plots are regions where values are not

available or not possible. Fig. 5 shows

the property plot of the liquidus temperature

vs. the alumina content. The

grey regions show where the alumina

concentration is smaller than the anode

effect concentration (on the left

side) or greater than the maximum

solubility of alumina in the electrolyte.

Electrolyte and aluminium

densities

Fig. 6: Aluminium and Electrolyte Densities Plot Pane. The

user has clicked on the aluminium fluoride icon-plot, so that

the property plot shows the aluminium and electrolyte density

vs. the aluminium fluoride concentration.

As an example the following chapter

discusses the densities of the electrolyte

and of the liquid aluminium

metal. In the electrolytic production

of aluminium the density values especially

the density difference between

aluminium and electrolyte affects

the separation of the produced

aluminium metal from the bath. This

difference should be larger than 0.2

g/cm 3 in order to prevent mixing and

to maintain good separation between

the metal pad and the electrolyte layer.

In the electrolyte and aluminium

density plot pane AlPrg shows simultaneously

the densities of the liquid

aluminium metal and the liquid electrolyte.

In this way the user sees immediately

the influence of parameter

changes on the density difference.

Dragging

If the mouse pointer is over the property

plot and the user presses the right

mouse button a so-called value line

Fig. 7: Dragging of the Value Line. When the user holds down the right mouse button AlPrg

draws the so-called value line. When the user drags this line by moving the mouse, AlPrg

registers a new input value and plots new density values in the property and icon plots.

is drawn. When the user displaces

the mouse (dragging) the value line

follows this motion on the property

plot. AlPrg registers this new concentration

input value (5% aluminium

fluoride in Fig. 7) and calculates the

corresponding new density values in

the icon and property plots (and electrolyte

composition/properties page).

If the user wants to change another

parameter, the lithium fluoride concentration,

the procedure is the same.

The user clicks on the lithium fluoride

icon plot. This icon plot is then selected,

i.e. AlPrg shows this plot in the

main property plot. By again dragging

the value line with the mouse pointer,

the user changes then the lithium

fluoride concentration,

and AlPrg calculates the

corresponding density

values.

Temperature plots

The superheat, i.e. the

temperature difference

between the electrolyte

temperature and the

liquidus temperature, is

a very important value for everyday

pot operation. Sometimes some quite

puzzling events are observed, namely

that the superheat might be negative

(the electrolytic bath should be

solid) even for a rather long time period.

This effect is called the liquidus

enigma [10].

If the superheat (ΔT) is selected as

input value on the electrolyte composition/properties

page (see Fig. 2, 3

and 4), then the electrolyte composition/properties

diagram pane shows

two icon-plots, namely the superheat

plot (densities vs. superheat on Fig.

6, 7) and the electrolyte temperature

plot (bath temperature vs. aluminium

fluoride on Fig. 6, 7).

Clicking on the superheat iconplot

(ΔT) AlPrg draws the corresponding

property plot, namely aluminium

and electrolyte density vs. superheat.

By dragging the value circles, the user

can modify the input value of the superheat.

Since the last selected composition

icon plot was the densities vs.

aluminium fluoride icon-plot, AlPrg

shows in the bath temperature icon

plot how the electrolyte temperature

depends on aluminium fluoride concentration.

Fig. 9: Property Plot of Densities vs. Superheat. The user has

selected the superheat icon-plot of Fig. 6 and so AlPrg draws

the corresponding property plot densities vs. superheat.

If the electrolyte temperature is

selected as input value on the electrolyte

composition/properties page

(see Fig. 2, 3 and 4), then the electrolyte

composition/properties diagram

pane shows the densities vs. electrolyte

temperature icon-plot (Fig. 11). By

clicking on this icon plot, the user instructs

AlPrg to show the corresponding

aluminium and electrolyte density

vs. bath temperature property plot, as

well as the electrolyte temperature

plot (bath temperature vs. aluminium

fluoride on Fig. 6, 7).

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Fig. 10: Property Plot of Bath Temperature vs. Aluminium Fluoride Concentration. The user

has selected the bath temperature icon-plot of Fig. 6, and so AlPrg draws the corresponding

property plot bath temperature vs. aluminium fluoride concentration.

Conclusions

This paper describes the calculation

page and the diagram pane of the electrolyte

properties module of AlPrg. In

the calculation page the user enters

or modifies in a rather conventional

way values in the corresponding input

fields, and AlPrg then determines the

corresponding electrolyte properties,

like liquidus temperature, electrical

conductivity etc. The diagram pane

shows graphical representations, i.e.

small icon-plots, of how a selected

property (the liquidus temperature,

for instance) depends on the electrolyte

composition values and the

bath temperature. The user selects

an icon-plot to be drawn in the larger

and more precise property plot. This

plot is a graphical user interface, i.e.

the user can change input values by

dragging the so called values line with

the mouse pointer to a new position

and value.

The user investigates in this way

the behaviour not only of those electrolyte

properties already described in

this paper (liquidus temperature and

densities), but also the electrical conductivity

(interesting for pot voltage

settings), maximal alumina solubility

(used for point feeder setting) , vapour

Fig. 11: Densities vs. Electrolyte Temperature Property Plot. The user has selected the electrolyte

temperature as input value on the electrolyte composition/properties page. AlPrg

then shows the densities vs. electrolyte temperature icon- and property plot after appropriate

selection by the user. The shaded area is the region where the bath temperature is

lower or higher than the liquidus temperature +/- the maximum superheat (±50 °C).

pressure (necessary for environmental

issues) and the viscosities of the

liquid aluminium pad and electrolyte

(used for hydrodynamic studies).

This paper describes the electrolyte

properties module as a standalone

programme. This module is

however also incorporated into other

modules where it is needed, for instance

into pot voltage or the profitability

module. It works there in the

same interactive way as described in

this paper; this means the user can

change an input value either from the

key board on the electrolyte property

page, or else by dragging on the corresponding

property plot. AlPrg then

recalculates other parameters and

updates the corresponding pages or

plot panes.

References

[1] P. M. Entner, “New Feature of ElyseSem/

ElysePrg”, Aluminium 75 (1999) 12, 1064-

1072

[2] http://www.peter-entner.com.

[3] A. Solheim, S. Rolseth, E. Skybakmoen,

L. Støen, Å. Sterten, T. Støre, “Liquidus

Temperature and Alumina Solubility in

the System Na 3

AlF 6

-AlF 3

-LiF-CaF 2

-MgF 2

”,

Light Metals (1995), 451-460.

[4] A. Røstum, A. Solheim, A. Sterten, “Phase

Diagram Data in the System Na 3

AlF 6

-Li-

AlF -AlF -Al O 3 6 3 2 3

. Part I: Liquidus Temperatures

for Primary Cryolite Crystallisation”,

Light Metals (1990), 311-316.

[5] R. D. Peterson, A. T. Tabereaux, “Liquidus

Curves for the Cryolite-AlF 3

-CaF 2

-

Al 2

O 3

System in Aluminum Cell Electrolytes”,

Light Metals (1987), 383-388.

[6] G. L. Bullard, D. D. Przybycien, “DTA Determination

of Bath Liquidus Temperatures:

Effect of LiF”, Light Metals (1986), 437-443.

[7] S. S. Lee, K.-S. Lei, P. Xu, J. J. Brown

Jr., “Determination of Melting Temperatures

and Al 2

O 3

Solubilities for Hall Cell

Electrolyte Compositions”, Light Metals

(1984), 841-855.

[8] E. W. Dewing, “The Chemistry of the

Alumina Reduction Cell”, Can. Metallurgical

Quarterly (1974), 13 (No.4), 607-618;

JOM 53 (2001) 5, pp. 29-34.

[9] http://www.peter-entner.com/e/theory/thcontents.htm.

[10] B. P. Moxnes, A. Solheim, T. Støre, B.

E. Aga, L. Støen, “The 'Liquidus Enigma'

revisited”, Light Metals (2006), 285-290.

Author

Peter M. Entner has studied chemistry at

the University of Vienna. He pursued his

studies at the University of Pennsyslvania

and University of Geneva (high temperature

electrochemistry and crystallography).

He joined then Alusuisse that later became

Alcan to work on research and development

projects about aluminium smelting.

Being retired, Peter’s passion for bytes and

pixels keeps him busy to work on software

that might be useful for the members of the

aluminium smelting community.

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